Hi Sherlockian and welcome to the WAB forum.
This website harbours all manner of folks, with views on wildlife that cover a wide spectrum ... not all of which might be considered to be particularly enlightened or even especially wildlife-friendly!
So don't assume my response to be anywhere close to typical ... I expect that others, with viewpoints diammetrically opposed to my own, will make themselves known to you also.
First of all, let me ask you to consider that the property you have recently moved into and imagine to now be 'yours' ... is actually only so because you have signed a contract with another human being ... and not because you have entered into some covenant with the Universe, Mother Nature, God, or whatever other higher authority, depending on your personal belief system. We share this planet with lots of other life-forms ... it's not ours exclusively.
This property that has lain empty (of humans) for 2 years, may have then already been home to many generations of rats and other small occupants ... and perhaps for countless more generations on that same plot in the years before the house was even erected.
So just who are the real interlopers in this story ... the rats, or you and your family?
I'm only asking you to alter your viewpoint and perceptions for the sake of fairness and justice ... two of the key things which afterall underpin our concept of advanced civilization.
Are you justified in trying to kill the rats to end your perceived conflict with them? Not in my opinion ... and certainly not by deploying means that are clearly not 'humane' if the traps you used are causing injury and prolonged suffering, and not an instant death. Rat poison always causes prolonged suffering, as the animals slowly bleed to death through every orifice ... and poses a further risk to many things that may consume the carcase.
Your situation is not without a satisfactory resolution (for all parties) however, if you choose to take an enlightened approach ... and by understanding something about rats.
Rats are intelligent mammals, but they are naturally nervous of anything new ... they especially don't like disturbance of their established routines.
It may be enough to encourage the rats to vacate your loft, by simply using it on a regular basis ... but more practically and on a shorter term basis, you can encourage them to look for more suitable accommodation by putting in the loft (or other affected places) :
a) a battery-powered radio, tuned to a music or talk channel (though you'd probably want to turn this off overnight, which may diminish it's effectiveness

)
and/or
b) a battery-powered strobe-light
Suitable strobe-lights are those often found in mountaineering outfitters, or ship chandlers (search online or find thenm on eBay), for use as personal emergency beacons. They emit a very bright flash of light every few seconds and can run continuously for more than 16 hrs on a single D-cell battery (rechargeable ones are more cost-effective in the longterm).
This method of encouraging rats or mice to vacate 'your' property involves only a low-cost outlay, is humane, hygenic ... and quickly effective !
However, you can ONLY use this method, if you are absolutely sure that your loft-space isn't also providing safe-harbour to bats, pine martens, or any other specially protected species (that you should also feel honoured to have sharing where you choose to make your home).
Once the rats have left, you then need to ensure you fit adequate barriers to discourage their return.